The present invention relates to electrical energy storage systems which may be recharged over numerous cycles to provide reliable power sources for a wide range of electrical utilization devices. The invention is directed in particular to a rechargeable storage system which is capable of exhibiting both high energy density normally associated with batteries, and high power density and long operative life typical of supercapacitors.
In the present invention, such a system comprises a multi-layer energy storage device structure which incorporates respective positive and negative electrode elements comprising pseudocapacitor or double-layer supercapacitor materials and rechargeable intercalation battery materials in a unitary, flexible structure which may be sized and shaped as desired to be most compatible with utilization devices while providing advantageously high energy and power densities.
Modern applications requiring mobile electrical energy sources, ranging from personal telecommunications devices to electric vehicles, are proliferating at an exponential rate. The demands of these applications range widely, for example, in voltage or power level, but all are preferably served by light-weight storage devices which can rapidly provide consistently high energy density over long time spans and can be quickly recharged to operational energy levels. Commercially, these extensive mobile energy needs are being met, in a fashion, by one or the other of the two available types of storage devices, viz., rechargeable batteries, such as lithium-ion intercalation systems, or supercapacitors of either faradic pseudocapacitive or non-faradic double-layer reaction type.
The choice between these battery or supercapacitor systems is normally dictated by the more pressing of the application""s demand for high energy density, available from batteries, or for the rapid delivery of high power, provided by supercapacitors. Attempts to meet requirements for both high energy and high power densities in a single application have led in some instances to the utilization of both device types arranged together in such a manner that the battery is available to recharge the supercapacitor between periods of high power demand. The disadvantage of such a practice in the excessive weight factor alone is clearly apparent. Additional limitations on this expedient are reflected in the time requirement for battery charging, as well as in the multiplicity of cells and in battery life cycle which may often be shortened by the physical rigors of the intercalation battery charging operation.
Recently, inroads have been made toward meeting the requirements of mobile electrical energy utilization in a hybrid system which combines the desirable characteristics of both the battery and the supercapacitor in a single integrated device of light weight and extended energy capacity. Comprising opposing electrodes of, for example, an activated carbon supercapacitor element and an intercalation composition battery element, particularly a transition metal oxide spinel material having a structure which exhibits rapid ion diffusion and little physical distortion from intercalation, such a hybrid system is able to exhibit a higher energy storage capability more typical of batteries as well as the high speed power delivery and exceptional cycle life of supercapacitors. The present invention represents an improvement upon this hybrid system, particularly in enabling a manyfold increase in achievable energy density without loss of contemporary high power density and rapid cycling capability.
A hybrid battery/supercapacitor system embodying the improvement of the present invention comprises, in essence, negative and positive electrode members with an interposed insulative ion-transmissive separator member containing a fluid electrolyte. These functional members are preferably in the form of individual layers or membranes laminated together to form a flexible, unitary structure.
The negative xe2x80x9cbatteryxe2x80x9d electrode member layer comprises a composition of a material capable of reversibly intercalating cation species, preferably comprising a spinel compound dispersed in a polymeric matrix of, for example, a copolymer of poly(vinylidene fluoride-co-hexafluoropropylene). To provide low resistance electrical current conduction between electrodes, the battery layer may be laminated, e.g., by thermal means, to a conductive current collector element, such as a reticulated metal foil.
The positive xe2x80x9csupercapacitorxe2x80x9d counter-electrode member layer may be similarly combined with a current collector foil and preferably comprises a copolymer matrix composition of activated carbon and, in accordance with the present invention, an added intercalation material, such as a spinel compound, incorporating a reversibly intercalated cation species.
The separator member interposed between the electrode members may comprise any of the previously employed high-porosity, microporous, or absorptive polymer film layers or membranes within which is dispersed a solution of electrolyte salt comprising an intercalatable cation, e.g., 1 M solution of LiPF6 in a mixture of 2 parts ethylene carbonate and 1 part dimethyl carbonate. Such an electrolyte ensures essential ionic conductivity and mobility within the system structure. In the present invention this mobility serves the notable purpose of enabling the rapid flow of both ion species of the electrolyte salt to and from the respective electrodes during charging and discharging of the device.
In the hybrid system of the present invention, as in the operation of the prior hybrid system, the cation migration to intercalation within the negative electrode during a charging cycle, the action which normally serves as the sole mode of energy storage in rechargeable intercalation battery systems, is augmented by anion migration from the electrolyte to the positive electrode surface to effect an additional anion-adsorptive capacitive charging, e.g., of the non-faradic double-layer type. The combined effect of faradic intercalation battery charging and non-faradic capacitor charging rapidly builds an effective energy density which may be recovered at an equally rapid rate, e.g., about an order of magnitude faster than achievable in a typical Li-ion battery, to yield high power density upon application demand. Unfortunately however, the ultimate energy density levels of such earlier hybrid system have lagged by a factor of about three those of rechargeable Li-ion batteries.
During attempts to formulate a hybrid system capable of yielding improved energy densities approaching those of the rechargeable Li-ion battery systems, it was surprisingly discovered that the sought improvement could be realized by the addition of a lithiated intercalation material to the activated carbon positive electrode composition in a lithium cation electrolyte hybrid system. This advantageous result apparently derives from the additional Li+ ions which are provided by the positive electrode intercalation composition during the charging cycle of the system and which supplement the Li+ ions normally available from the dissociated electrolyte solute for energy storage intercalation into the composition of the negative electrode. The redox activity in the intercalation of the supplemental Li+ ions at the positive electrode during desorption of the electrolyte anions thereafter provides the additional electron flow yielding the exceptionally higher energy density levels which have been seen to exceed those of popular NiCd and NiMeH cells, as well.
The hybrid system of the present invention, as did the earlier hybrid system upon which it has improved, can utilize most of the respective compositions of known rechargeable intercalation batteries and supercapacitor devices, such as are typically represented, e.g., in U.S. Pat. Nos. 5,418,091 and 5,115,378. As in these earlier systems, intercalating electrode materials may comprise metallic sulfides, oxides, phosphates, and fluorides, open-structured carbonaceous graphites, hard carbons, and cokes, and alloying materials, such as aluminum, tin, and silicon. Similarly, surface-active capacitor materials, typically high surface area closed-structure activated carbon powders, foams, fibers, and fabrics may be used in the counter-electrodes. The active electrolyte component of the present hybrid system may likewise be formulated of prior available materials, with particular utility being enjoyed in the non-aqueous solutions of intercalatable alkali and alkaline earth cations, usually incorporated in significantly fluid form in fibrous or polymer matrix containment materials, thus maintaining an environment conducive to mobility of both species of electrolyte ions. The laminated polymeric layer format typified by the secondary batteries described in U.S. Pat. No. 5,460,904 and related publications serves well for the structures of the present invention.